This invention relates to a method of forming a contact metallization to a doped silicon region on a silicon substrate in a semiconductor. Specifically, this invention relates to methods using a titanium nitride (TiN) layer.
A prior art contact metallization method is described with respect to FIG. 1. A silicide layer is formed on the semiconductor wafer comprising the silicon substrate 2 with field oxide regions 4 and doped silicon regions 4. The silicide layer 8 is formed by sputtering a titanium layer off of a titanium target so that titanium is deposited on the wafer.
After sputtering the titanium on to the wafer, the wafer is put into a furnace or a rapid thermal processor. The titanium forms a silicide layer 8 when the titanium reacts with the silicon. The silicide layer typically has the formula TiSi.sub.2, but this silicide layer 8 may also contain intermediate compounds.
The titanium that is on the field oxide regions 4 will not react to form silicide. This titanium on the field oxide regions would short the semiconductor chip if not removed. The undesired titanium on top of the field oxide 4 is removed with a wet etch before an interlevel dielectric is placed on top of the wafer.
The silicide layer 8 is necessary because titanium nitride has a high contact resistance with the silicon in the doped silicon region 6. The silicide layer 8 has a low contact resistance with the doped silicon region 6.
A layer of an interlevel dielectric such as boro-phosphorous silicate glass or BPSG 10 is deposited over the top of the wafer. Contact holes are created in the BPSG layer 10 and typically, the BPSG layer 10 is heated so that it reflows to form a smooth surface. A layer of titanium nitride 12 is then formed on top of the BPSG layer 10 and the silicide layer 8. The titanium nitride layer 12 is formed by sputtering titanium from a titanium target in a chamber containing nitrogen gas so that the titanium reacts with the nitrogen gas to form titanium nitride. This titanium nitride forms a layer over the BPSG layer 10 and the silicide layer 8. The titanium nitride (TiN) layer 12 has a low contact resistance with the silicide layer 8. The titanium nitride layer 12 is used as a barrier metal to prevent spiking and epitaxial silicon growth in the contact hole. Spiking may cause a sputtered aluminum layer to drive through the doped silicon junction.
A metal layer including aluminum 14 is then sputtered on top of the titanium nitride layer 12. The metal layer 14 includes aluminum and may include other alloying elements. The titanium nitride layer 12 has a low contact resistance with the metal layer including aluminum and the silicide layer. The titanium nitride layer has a high electromigration resistance.
In the contact metallization, there is a problem with electromigration. Electromigration occurs when the cross-section of the metallic layer is small and therefore the current density going through the metallic layer is high. This high current density causes kinetic energy to transfer to the metal atoms which migrate away from the narrow cross-section. Faults caused by electromigration are problematic because they might not be found when the semiconductor chip is tested after production, but could occur after the testing when sufficient current has been put through the aluminum layer 14. The titanium nitride layer 12 resists the electromigration of the metal atoms.
Since the metal layer including aluminum 14 has a low contact resistance with the titanium nitride layer 12, and the silicide layer 8 also has a low contact resistance with the titanium nitride layer 12, the total contact resistance of the junction is low. The silicide layer 8 is necessary because of the high contact resistance between the doped silicon region and the titanium nitride layer.
This prior art process, however, does have disadvantages. The cost and time needed to produce a silicon chip depend upon the number of steps involved. It is therefore an object of the present invention to have a method of forming a titanium nitride contact metallization using fewer steps than the prior art methods.